In machine shop practice, it is customary to use a micrometer to measure the outside diameter of a part having a circular cross section, e.g. a cylindrical shaft, as different steps of the machining process are completed. Difficulties may be encountered in accurately and reliably placing the micrometer in position on the part to be measured, itself a time-consuming procedure, and occasionally errors in measurement occur. Furthermore, a particular micrometer can generally only measure a given range of cross sectional sizes, so that a number of micrometers must be kept on hand for measuring parts of widely different diameters. Such a requirement not only increases the amount of capital tied up in equipment, but it also necessitates a certain amount of record keeping and equipment storage. Finally, the maintenance costs of the shop are increased since each micrometer must be periodically re-calibrated.